It is a fraction that describes the amount of squeeze in an engine.
With the piston all the way down at the bottom, you measure the amount of volume in the cylinder. The piston is down in the bore and it has a "cylinder" of volume above it along with a combustion chamber worth of volume plus the volume created by the gasket thickness and the volume created (or removed) by the valve reliefs/dome etc. The total volume is the numerator (upper number) of the fraction.
The denominator (lower number) of the fraction is that same numerator volume minus the volume (created by the piston movement) that is removed when the piston is at the top of the stroke. You have the combustion chamber volume plus the volume created by the gasket thickness and the valve reliefs/dome etc.
The compression ratio is the number you get when you punch this fraction into your calculator. The higher the compression ratio, the higher the efficiency (POWER) up to the limitations of the fuel octane, all else being equal.
More "squeeze" is better until it starts to detonate. The static C/R is a measurement of squeeze. [img]smile.gif[/img]